Atomic layer materials, or ALMs, are an exciting new class of materials that come in stacked sheets of atoms with very strong in-plane bonds, but much weaker out-of-plane bonds (see figure 1). These layers can be isolated, resulting in sheets between one and a few atoms thick, and is as close to a truly two-dimensional system as we can achieve. Confining electrons to two dimensions give rise to exciting new physics which can be easily observed in shifts in electronic behavior as an ALM material is thinned down from a bulk sample. ALMs offer interesting physics due to their low dimensionality and have been shown to be useful in creating highly efficient photodetectors, integrated circuits, and other opto-electronic devices.

The amount of different ALMs at this point is far too numerous to list, but far and away the most prominent one is graphene (Figure 2), which consists of a single sheet of carbon atoms arranged in a hexagonal lattice. Not only is it the thinnest membrane possible, it also displays incredible electrical, thermal, and structural properties. These properties are also highly thickness-dependent: monolayer graphene behaves much like a metal, although strictly speaking it is not. Bilayer graphene, on the other hand, has properties resembling a semiconductor.

Another class of ALMs are the transition metal dichalcogenides (TMDs), which our lab heavily focuses on. Two-dimensional transition metal dichalcogenides (TMDs) have captured a great deal of attention in the scientific communities for their wide range of unique properties, as well as their promise for electronic and optoelectronic performance at the nanoscale.

These materials consist of one transition metal atom, such as W (Tungsten )and MO (Molybdenum), and two chalcogen atoms like Selenium(Se) , Sulfur(S) , and Tellurium (Te). such that one hexagonally packed layer of transition metal atoms is sandwiched between two layers of chalcogen atoms.

The bulk of TMDs consist of stacked sheets of atomically thin layers, each layer typically with a thickness of 6~7 Å. The intralayer bonds are predominantly covalent in nature, whereas the sandwich layers are coupled by weak van der Waals forces. The van der Waals interlayer interaction enables the possibility to exfoliate and stack different 2D materials into arbitrary and vertically heterostructures. Exfoliation of these materials into atomically thin two- dimensional crystals preserves their bulk properties, while new physical properties emerge due to quantum confinement effects.

The heterostructures formed by semiconductor TMDs play an important role in modern semiconductor industry as essential building blocks for electronic devices such as solar cells, photo-detectors, semiconductor lasers, and etc. When combined into heterostructures transition metal dichalcogenides band structure changes and forms a new band at the interface of the two materials.

For instance, the most interesting aspect has found in WTe₂. This non-magnetic layered transition-metal dichalcogenide has extremely large non-saturating positive magnetoresistance at low temperatures(Figure3). In addition, WTe₂ can works as a sensitive mid-IR detector for unusual applications in bio-inspired neural networks, which our group focused on it recently.

At the QMO lab we have studied the electronic transport characteristics at various regimes of bias and gate voltage in a heterostructure composed of two semiconductor TMD materials MoSe₂ and WSe₂. Through interlayer I-VSD characteristics we report on highly efficient multiplication of interlayer e- h pairs in these 2D semiconductor heterostructure photocells, indicating that layer- indirect e-h pairs are generated by hot electron impact excitation at temperatures near T = 300 K. By exploiting this highly efficient interlayer e-h pair multiplication process, we demonstrated near-infrared optoelectronic devices that exhibit 350% enhancement of the optoelectronic responsivity at microwatt power levels.

There are multiple ways to produce ALMs or TMDs. The two most common methods are mechanical exfoliation and chemical vapor deposition (CVD). In QMO lab the whole process need to take place in a glove box for WTe₂ since this material is extremely sensitive to the air.

At the QMO lab the heterostructure devices are assembled using two highly customized, temperature-controlled transfer microscopes one inside the glove box and one outside. The microscope inside is controlled from outside the glove box by different software’s on computer. The transfer stages ensure that the interface between the two layers has no intentional contact to polymer films.